The present invention relates to a thin film forming apparatus, and more specifically, the invention relates to a thin film forming apparatus by a plasma chemical vapor deposition (hereinafter referred to as CVD), which is suitable for manufacturing a magnetic disc.
A magnetic disc for a hard disc memory is normally made by forming a magnetic layer on a disc, such as a glass, and further providing a protection layer thereon. Conventionally, the protection layers are formed as carbonaceous protection films simultaneously on both surfaces of the disc by using a thin film forming apparatus by sputtering. However, as a new market demand, carbonaceous protection films formed by using a film forming method by a CVD method rather than a sputtering method have been requested.
However, in case the thin films are formed by a plasma CVD apparatus, the thicknesses of the formed films vary in accordance with the shape of a base plate (disc) holder. Therefore, there is a problem that a uniform film thickness can not be obtained.
Namely, conventionally, several uneven film thickness portions are formed partially in a disc 13 as shown in FIG. 4(b). The shape of a conventional base plate holder is one of the reasons of this problem. In particular, since a conventional base plate holder 18 is substantially rectangular as shown in FIG. 5, spaces between the disc 13, i.e. the base plate, and the base plate holder 18 are unequal as shown as A and B in FIG. 5. Corresponding to the vicinity of the portion A where a space between the base plate and the base plate holder is large, there is formed a portion in which a film forming speed is fast, that is, a thick film portion 34 where a thickness of the formed film is large.
In view of the above problem, an object of the invention is to provide a thin film forming apparatus by a plasma CVD, wherein a thin film with an equal thickness can be formed on surface of the base plate.
Another object of the invention is to provide a thin film forming apparatus as stated above, wherein the thin films with an equal thickness can be formed on both surfaces of the base plate at the same time.
Further objects and advantages of the invention will be apparent from the following description of the invention.